The human spine is composed of several vertebrae and an intervertebral disk. The intervertebral disk is a tissue containing a large amount of water and can absorb the vertical pressure of the spine and provide cushioning effect between upper and lower vertebrae. When any sections of the intervertebral disk change pathologically or due to aging, pain will occur due to compression of the adjacent spinal nerve, resulting in impaired movement.
Regarding the aforementioned problems resulting from injury of the intervertebral disk, conventional techniques offer an approach of implanting an artificial spinal interbody cage. However, the height of the conventional spinal interbody cage is fixed and thus constrains any attempt to minimize wound size.
FIG. 6 shows an adjustable spinal interbody cage employing conventional technique, which is disclosed in the publication (No. 2008/0140207) of a US patent. The conventional spinal interbody cage comprises an actuation shaft 4000, an upper plate 1000, a lower plate 2000 and two wedge members 3100 and 3200. The wedge members 3100 and 3200 upwardly approach each other and contact the upper plate 1000 and the lower plate 2000 in the direction of the axis of the actuation shaft 4000 to force the upper plate to part from the lower plate so as to adjust the height of the spinal interbody cage. However, when the height of the conventional spinal interbody cage is being adjusted, one end of the actuation shaft 4000 will protrude from the longitudinal ends of the upper plate 1000 and the lower plate 2000; such structure will increase the volume of the object implanted between the sections of the vertebra. When the spinal interbody cage is disposed between the sections of the vertebrae with the height adjusted as shown in FIG. 5, the protruding end of the actuation shaft is likely to cause compression to the neighboring nerves or tissues of the sections of the vertebrae, and can also complicate the surgery necessary to implant the spinal interbody cage.